1. Field of the Invention
This invention relates to a stamper holder that is mounted in a mold of a mold assembly, together with a stamper, for forming a substrate for an information recording medium, a mold component that includes the stamper holder and the stamper and is mounted in the mold, and a mold assembly in which the mold component is mounted.
2. Description of the Related Art
Conventionally, as a mold assembly for molding a substrate for an optical recording medium, such as a CD (Compact Disc) and a DVD (Digital Versatile Disc), Japanese Laid-Open Patent Publication (Kokai) No. H09-123229 discloses a mold assembly that includes a fixed mold having a stamper set therein for forming micro asperities, such as grooves, in a surface of the substrate, and a movable mold which cooperates with the fixed mold to define a cavity as a substrate-molding space.
The mold assembly is comprised of the fixed mold, the movable mold, and a peripheral mold. The fixed mold has a cavity-defining section partitioned on one side thereof, for forming one major surface of a disk-shaped substrate (base) on one side thereof. The movable mold is disposed in opposed-relation to the fixed mold and configured such that it is movable toward and away from the fixed mold by a guide means and a drive means. Further, the movable mold has a cavity-defining section partitioned on a side thereof opposed to the fixed mold, for forming the other major surface of the substrate on the other side. The fixed mold and the movable mold are mounted to mold-mounting parts of an injection molder. Further, the peripheral mold is associated with the movable mold, for molding a peripheral part of the substrate. The fixed mold, the movable mold, and the peripheral mold are clamped to cooperatively define the cavity as the substrate-molding space.
Further, the cavity-defining section of the fixed mold formed on the side thereof opposed to the movable mold, for defining the cavity, has as a smooth flat surface. The cavity-defining section has a stamper mounted thereon. In this case, the stamper has minute asperities formed in a major surface thereof facing the cavity, for forming an information signal-recording portion on the one major surface of the substrate. The stamper is in the form of a disk having a central hole formed through a central portion thereof, and mounted in the fixed mold using an inner periphery-side stamper holder and an outer periphery-side stamper holder. More specifically, the inner periphery-side stamper holder of the two stamper holders has a generally hollow cylindrical shape having an outer diameter approximately equal to the inner diameter of the central hole of the stamper, and at the same time has a wedge-shaped engaging portion integrally formed on its outer peripheral edge in a manner protruding therefrom. Therefore, the engaging portion of the inner periphery-side stamper holder is caused to be opposed to and engaged with the inner peripheral wall of the central hole of the stamper, whereby the stamper has its inner peripheral portion held by the inner periphery-side stamper holder, for being mounted in the fixed mold.
However, the present inventors studied the stamper holder and the mold assembly, and found out the following problems: As disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H09-293279, when the central hole of the stamper is formed by a punching machine, the inner peripheral surface of the central hole is in the form of a tapered surface. Therefore, as shown in FIG. 9, in the case of the conventional mold assembly 100, when the stamper 119 is held by the inner periphery-side stamper holder 118, the inner peripheral surface A of the stamper 119 and the outer peripheral surface B of the inner periphery-side stamper holder 118 are in line contact with each other via respective portions thereof indicated by a symbol C. On the other hand, when the substrate (base) is molded, a high-temperature resin is injected into the cavity, so that the stamper 119 repeatedly undergoes thermal expansion and thermal contraction. In the state where the contact area between the stamper 119 and the inner periphery-side stamper holder 118 is small, the stamper 119 is displaced to be made off-center when the high-temperature resin is injected into the cavity, so that the deviation (eccentricity) of the center of the substrate from the center of the grooves formed in the major surface of the substrate increases. Further, the small contact area makes the inner peripheral surface of the stamper 119 easy to wear, so that as wear occurs, the displacement of the stamper 119 becomes marked. Therefore, this stamper holder 118 suffers from the problem that due to the displacement (eccentricity) of the stamper 119 during molding, the quality of the substrate as the molded product is degraded.
On the other hand, a mold assembly 61 is also known in which a stamper 64 is mounted in a fixed mold 63 using a stamper holder 62 having no engaging portion (designated by a symbol D in FIG. 9), as shown in FIG. 10. In this mold assembly 61, however, the resin material enters a gap between the inner peripheral surface A of the stamper 64 and the outer peripheral surface B of the stamper holder 62. Therefore, this mold assembly 61 suffers from the problem that burrs are formed on the surface of the substrate 65 to degrade the quality of the substrate 65.